Communication apparatus and communication method

ABSTRACT

A communication apparatus and a communication method capable of suppressing an increase of bits used for a request to send a reference signal and flexibly setting a resource used for sending a reference signal. In a base station, a transmission processing unit transmits, in one of a plurality of formats, control information containing a request to send a sounding reference signal (A-SRS), and a reception processing unit receives the transmitted A-SRS using the resource specified by the format of the transmitted control information. Then, the plurality of formats is associated with each different SRS resource by a setting unit.

BACKGROUND

Technical Field

The claimed invention relates to a communication apparatus and acommunication method.

Description of the Related Art

The 3rd Generation Partnership Project Radio Access Network Long TermEvolution (hereinafter, referred to as LTE) (3GPP-LTE) employsorthogonal frequency division multiple access (OFDMA) for the downlinkcommunication scheme and single carrier frequency division multipleaccess (SC-FDMA) for the uplink communication scheme (see, NPLs 1, 2 and3, for example). Moreover, periodic sounding reference signals (P-SRS)are used in the uplink of LTE as reference signals for estimating theuplink reception quality.

In order to transmit P-SRS from a terminal to a base station, an SRStransmission subframe which is common to all terminals (hereinafter,referred to as common SRS subframe) is configured. This common SRSsubframe is defined by a combination of a predetermined periodicity anda subframe offset on a per-cell basis. In addition, the information onthe common SRS subframe is broadcasted to terminals within the cell. Forexample, when the periodicity is equal to 10 subframes and the offset is3, the third subframe in a frame (consisting of 10 subframes) isconfigured as a common SRS subframe. In a common SRS subframe, all theterminals within the cell stop transmission of data signals in the lastSC-FDMA symbol of the subframe and use the period as the resources fortransmission of reference signals.

Meanwhile, subframes for SRS transmissions are individually configuredfor terminals by a higher layer (i.e., RRC layer higher than thephysical layer) (hereinafter, referred to as individual SRS subframe).Each terminal transmits P-SRS in the configured individual SRS subframe.In addition, parameters for SRS resources (hereinafter, may be referredto as “SRS resource parameters”) are configured and reported to eachterminal. The parameters for the SRS resources include the bandwidth,bandwidth position (or SRS frequency domain starting position), cyclicshift and comb (corresponding to identification information on thesubcarrier group) of the SRS, for example. The terminal transmits SRSusing the resources specified by the reported parameters. Additionally,SRS frequency-hopping may be configured.

Meanwhile, the introduction of dynamic aperiodic SRS (hereinafter,referred to as A-SRS) into the uplink of LTE-Advanced, which is anadvanced version of LTE (hereinafter, referred to as “LTE-A”) has beendiscussed. The transmission timing of A-SRS is controlled by triggerinformation (e.g., 1-bit information). This trigger information istransmitted to a terminal from a base station on a physical layercontrol channel (i.e., PDCCH) (e.g., see NPL 4). To put it morespecifically, the terminal transmits A-SRS only upon request for A-SRStransmission made by the trigger information (i.e., A-SRS transmissionrequest). In addition, there has been discussion on defining, as thetransmission timing of A-SRS, the first common SRS subframe locatedafter the fourth subframe from the subframe in which the triggerinformation has been transmitted. As described above, while terminalstransmit P-SRS, periodically, terminals are allowed to transmit A-SRS ina concentrated manner within a short period only when uplink datatransmissions occur in bursts, for example.

Moreover, LTE-A has control information formats for various types ofdata assignment reporting. The control information formats in thedownlink include: DCI format 1A for allocation of resource blocksconsecutive in number (Virtual RBs or Physical RBs); DCI format 1, whichallows allocation of RBs not consecutive in number (hereinafter,referred to as “non-contiguous bandwidth allocation”); DCI formats 2 and2A for assigning a spatial-multiplexing MIMO transmission; a downlinkassignment control information format for assigning a beam-formingtransmission (“beam-forming assignment downlink format”: DCI format 1B);and a downlink assignment control information format for assigning amulti-user MIMO transmission (“multi-user MIMO assignment downlinkformat”: DCI format 1D). Meanwhile, the uplink assignment formatsinclude DCI format 0 for assigning a single antenna port transmissionand DCI format 4 for assigning an uplink spatial-multiplexing MIMOtransmission. DCI format 4 is used for only terminals in which uplinkspatial-multiplexing MIMO transmission is configured.

In addition, DCI format 0 and DCI format 1A are adjusted in size bypadding so that each format consists of the same number of bits. DCIformat 0 and DCI format 1A are also called DCI format 0/1A in somecases. DCI formats 1, 2, 2A, 1B and 1D are used in accordance withdownlink transmission modes configured in each terminal (i.e.,non-contiguous bandwidth allocation, spatial-multiplexing MIMOtransmission, beam-forming transmission and multi-user MIMOtransmission) and are formats to be configured in each terminal.Meanwhile, DCI format 0/1A can be used independently of the transmissionmodes and thus can be used for terminals in any transmission mode, i.e.,DCI format 0/1A is a format commonly usable in all terminals. Inaddition, when DCI format 0/1A is used, single-antenna transmission ortransmit diversity is used as the default transmission mode.

Terminals receive DCI format 0/1A and the DCI formats that are dependenton the downlink transmission modes. In addition, terminals in whichuplink spatial-multiplexing MIMO transmission is configured receive DCIformat 4 in addition to the DCI formats mentioned above.

In this respect, using DCI format 0 for reporting the triggerinformation for A-SRS has been discussed. DCI format 0 is a controlinformation format used in reporting uplink data (PUSCH) assignment. Thefield for reporting the trigger for A-SRS is added to DCI format 0 inaddition to RB reporting field, MCS reporting field, HARQ informationreporting field, transmission power control command reporting field andterminal ID field. It should be noted that, A-SRS and P-SRS can be usedtogether or singly. In addition, parameters for SRS resources (e.g.,transmission bandwidth, cyclic shift and/or the like) are configuredindependently for A-SRS and P-SRS.

CITATION LIST Non-Patent Literature

-   NPL 1 3GPP TS 36.211 V8.7.0, “Physical Channels and Modulation    (Release 8),”September 2008-   NPL 2 3GPP TS 36.212 V8.7.0, “Multiplexing and channel coding    (Release 8),”September 2008-   NPL 3 3GPP TS 36.213 V8.7.0, “Physical layer procedures (Release    8),” September 2008-   NPL 4 3GPP TSG RAN WG1 meeting, R1-105439, “Views on Signaling for    Dynamic Aperiodic SRS,” October 2010

BRIEF SUMMARY Technical Problem

When the above mentioned trigger information for A-SRS is represented bya single bit, the trigger information can be used to report two statesindicating a request for A-SRS transmission and no A-SRS transmission.In this case, all the SRS resource parameters (e.g., bandwidth, cyclicshift and/or the like) are reported semi-statically using higher layercontrol information (i.e., RRC signaling). Frequent reporting using RRCsignaling is not preferable in terms of the overhead for the controlinformation as well as the processing load on base stations andterminals. Accordingly, each terminal uses the configured SRS resourceparameters for a long period of time.

In this case, an assumption is made that each terminal uses the SRSresources previously configured by RRC signaling for a long period oftime when the trigger information for A-SRS is represented by a singlebit. Accordingly, when the trigger information is transmitted to aplurality of terminals, there is a possibility of collision between SRSstransmitted from a plurality of terminals in the same SRS transmissionsubframe. This possibility increases as the number of terminalsincreases. To avoid this collision, the SRS transmission timings of theplurality of terminals need to be varied. To put it more specifically,it is necessary to delay the A-SRS transmission timing of any of theterminals. For this reason, when the trigger information for A-SRS isrepresented by a single bit, the delay of A-SRS causes a reduction inthe accuracy of frequency scheduling in base stations, leading to areduction in the system throughput due to degraded accuracy in MCSselection.

Meanwhile, it is possible to configure the SRS resources in the units ofsubframes by increasing the number of bits representing the triggerinformation for A-SRS. For example, four states can be reported when twobits are used to represent the trigger information. The four statesherein include no A-SRS transmission, and requests for A-SRStransmission with cyclic shift 1 (i.e., transmission using SRS resource1), A-SRS transmission with cyclic shift 2 (i.e., transmission using SRSresource 2), and A-SRS transmission with cyclic shift 3 (i.e.,transmission using SRS resource 3). In this configuration, since theflexibility in configuring the SRS resources is increased to someextent, the probability of SRS resources being identical betweenterminals is reduced. Thus, the probability of collision between thetransmitted SRSs can be reduced. However, since the number of bits usedto represent the trigger information for A-SRS is increased, therearises a problem that the overhead for the control informationincreases.

An object of the claimed invention is to provide a communicationapparatus and a communication method that allow flexibly configuringresources used for transmission of reference signals while limiting anincrease in the number of bits used to request the transmission ofreference signals.

Solution to Problem

A communication apparatus according to an aspect of the claimedinvention includes: a receiving section that receives controlinformation in one of a plurality of formats, the control informationincluding a transmission request for sounding reference signals (SRS);and a transmitting section that transmits the SRS, using a resourceidentified by the format of the received control information, in whichthe plurality of formats are respectively associated with differentresource configuration numbers identifying the resources.

A communication apparatus according to an aspect of the claimedinvention includes: a transmitting section that transmits controlinformation in one of a plurality of formats, the control informationincluding a transmission request for sounding reference signals (SRS);and a receiving section that receives the SRS transmitted using aresource identified by the format of the control information, in whichthe plurality of formats are respectively associated with differentresource configuration numbers identifying the resources.

A communication method according to an aspect of the claimed inventionincludes: identifying a resource from a format of control informationincluding a transmission request for sounding reference signals (SRS)received in one of a plurality of formats; transmitting the SRS usingthe identified resource, in which the plurality of formats arerespectively associated with different resource configuration numbersidentifying the resources.

A communication method according to an aspect of the claimed inventionincludes: transmitting control information in one of a plurality offormats, the control information including a transmission request forsounding reference signals (SRS); and receiving the SRS transmittedusing a resource identified by the format of the control information, inwhich the plurality of formats are respectively associated withdifferent resource configuration numbers identifying the resources.

Advantageous Effects of Invention

The claimed invention can provide a communication apparatus and acommunication method that allow flexibly configuring resources used fortransmission of reference signals while limiting an increase in thenumber of bits used in a request for the transmission of referencesignals.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 is a main configuration diagram of a base station according toEmbodiment 1 of the claimed invention;

FIG. 2 is a main configuration diagram of a terminal according toEmbodiment 1 of the claimed invention;

FIG. 3 is a block diagram illustrating a configuration of the basestation according to Embodiment 1 of the claimed invention;

FIG. 4 is a block diagram illustrating a configuration of the terminalaccording to Embodiment 1 of the claimed invention;

FIG. 5 is a diagram provided for describing rules for A-SRStransmission;

FIG. 6 is a diagram provided for describing transmission of triggerinformation and A-SRS transmission;

FIG. 7 is a diagram provided for describing rules for A-SRS transmissionaccording to Embodiment 2;

FIG. 8 is a diagram provided for describing a variation of the rules forA-SRS transmission; and

FIG. 9 is a diagram provided for describing rules for A-SRS transmissionaccording to Embodiment 3.

DETAILED DESCRIPTION

Hereinafter, embodiments of the claimed invention will be described indetail with reference to the drawings. Throughout the embodiments, thesame elements are assigned the same reference numerals and any duplicatedescription of the elements is omitted.

Embodiment 1

(Overview of Communication System)

A communication system according to Embodiment 1 of the claimedinvention includes base station 100 and terminals 200.

Base station 100 is an LTE-A compliant base station and terminals 200are LTE-A compliant terminals.

FIG. 1 is a main configuration diagram of base station 100 according toEmbodiment 1 of the claimed invention. In base station 100, transmissionprocessing section 104 transmits control information including a requestfor transmission of sounding reference signals (hereinafter, “A-SRS”),using one of a plurality of formats, and reception processing section108 receives A-SRS transmitted using the resources identified by theformat of the transmitted control information. Configuration section 101associates the plurality of formats with different SRS resourceconfiguration numbers, respectively.

FIG. 2 is a main configuration diagram of terminal 200 according toEmbodiment 1 of the claimed invention. In terminal 200, receptionprocessing section 203 receives control information including a requestfor transmission of sounding reference signals (hereinafter, “A-SRS”),using one of the plurality of formats, and transmission signal formingsection 207 transmits A-SRS using the resources identified by the formatof the received control information. Transmission controlling section206 associates the plurality of formats with different SRS resourceconfiguration numbers, respectively.

Hereinafter, a description will be provided with an assumption that anFDD system, which separates the uplink and downlink into two frequencybands, is employed.

(Configuration of Base Station 100)

FIG. 3 is a block diagram illustrating a configuration of base station100 according to Embodiment 1 of the claimed invention. In FIG. 3, basestation 100 includes configuration section 101, coding and modulationsections 102 and 103, transmission processing section 104, RF (RadioFrequency) transmitting section 105, antenna 106, RF (Radio Frequency)receiving section 107, reception processing section 108, data receivingsection 109 and SRS receiving section 110.

Configuration section 101 generates “A-SRS transmission ruleconfiguration information” for configuring a correspondence between acontrol information format (e.g., DCI format) used for transmitting arequest for A-SRS to configuration target terminal 200, and resourcesused for the transmission of A-SRS from configuration target terminal200 (hereinafter, A-SRS resource).

The A-SRS transmission rule setting information includes identificationinformation on a plurality of control information formats (i.e., DCIformats) and information about A-SRS resources corresponding to theidentification information on each of the control information formats.These A-SRS resources are resources to which terminal 200 maps A-SRS asdescribed above.

The information about A-SRS resources includes parameters such as afrequency bandwidth (or the initial RB position of SRS bandwidth),bandwidth (or the number of RBs), cyclic shift, transmission comb, thenumber of antennas, the number of transmissions, frequency hopping andcomponent carrier. To put it more specifically, based on the A-SRStransmission rule configuration information, combinations each includethe identification information on one of the plurality of controlinformation formats (i.e., DCI formats) and the parameters correspondingto the identification information on the control information formats areconfigured for configuration target terminal 200.

In addition, configuration section 101 generates uplink assignmentcontrol information or downlink assignment control information includingtrigger information instructing instruction target terminal 200 totransmit A-SRS (hereinafter, simply referred to as “triggerinformation”).

As described above, the A-SRS transmission rule configurationinformation generated by configuration section 101 is transmitted toconfiguration target terminal 200 after the A-SRS transmission ruleconfiguration information undergoes transmission processing performed bycoding and modulation section 102, transmission processing section 104and RF transmitting section 105, as the control information of the RRClayer. In addition, the control information including the triggerinformation for A-SRS transmission is transmitted to configurationtarget terminal 200 after the control information undergoes transmissionprocessing performed by coding and modulation section 102, transmissionprocessing section 104 and RF transmitting section 105, as the controlinformation of the layers 1 and 2. The trigger information isrepresented by a single bit. When the bit is 0, this means that thetrigger information indicates the instruction to transmit A-SRS. Whenthe bit is 1, this means that the trigger information indicates no A-SRStransmission.

Configuration section 101 generates assignment control informationincluding resource (i.e., RB) assignment information and MCS informationfor one or a plurality of transport blocks (TB), as control informationincluding the trigger information.

The assignment control information includes assignment controlinformation on uplink resources for assigning uplink data (e.g.,physical uplink shared channel (PUSCH)) and downlink resources forassigning downlink data (e.g., physical downlink shared channel(PDSCH)). The assignment control information for assigning uplink datainclude DCI formats 0 and 4 and examples of assignment controlinformation for assigning downlink data include DCI formats 1A, 1, 1B,1D, 2 and 2A.

Configuration 101 transmits the A-SRS transmission rule configurationinformation to configuration target terminal 200 via coding andmodulation section 102 and also outputs the A-SRS transmission ruleconfiguration information to reception processing section 108. Inaddition, configuration section 101 transmits the assignment controlinformation including the trigger information to configuration targetterminal 200 via coding and modulation section 102 and also outputs theassignment control information to transmission processing section 104.Moreover, configuration section 101 outputs information indicating theformat (i.e., DCI format) of the assignment control informationincluding the trigger information.

Base station 100 reports the configuration information to terminal 200as higher layer information (i.e., using RRC signaling). Meanwhile, basestation 100 reports the assignment control information and the triggerinformation to terminal 200, using physical downlink control channel(PDCCH). To put it more specifically, while the reporting intervals ofthe configuration information are relatively long (i.e., theconfiguration information is reported between relatively longintervals), the reporting intervals of the assignment controlinformation and the trigger information are relatively short (i.e., theassignment control information and the trigger information are reportedbetween relatively short intervals).

Coding and modulation section 102 encodes and modulates theconfiguration information, trigger information and assignment controlinformation received from configuration section 101 and outputs theobtained modulation signals to transmission processing section 104.

Coding and modulation section 103 encodes and modulates data signals tobe received and outputs the obtained modulation signals to transmissionprocessing section 104.

Transmission processing section 104 forms transmission signals bymapping the modulation signals to be received from coding and modulationsections 102 and 103 to the resources indicated by the downlink resourceassignment information received from configuration section 101. In acase where the transmission signals are OFDM signals, OFDM signals areformed by mapping the modulation signals to the resources indicated bythe downlink resource assignment information to be received fromconfiguration section 101, transforming the mapped signals into a timewaveform by inverse fast Fourier transform (IFFT) and adding cyclicprefix (CP) to the resultant signals.

RF transmitting section 105 performs radio transmission processing(e.g., up-conversion, digital to analog (D/A) conversion and/or thelike) on the transmission signals received from transmission processingsection 104 and transmits the resultant signals via antenna 106.

RF receiving section 107 performs radio reception processing (e.g.,down-conversion, analog to digital (A/D) conversion and/or the like) onthe radio signals received via antenna 106 and outputs the obtainedreceived signals to reception processing section 108.

Reception processing section 108 identifies the resources to which theuplink data signals and ACK/NACK information are mapped, on the basis ofthe uplink resource assignment information received from configurationsection 101 and extracts a signal component mapped to the identifiedresources from the received signals.

In addition, reception processing section 108 identifies the resourcesto which A-SRS is mapped, on the basis of transmission ruleconfiguration information, the trigger information and the DCI formatinformation received from configuration section 101, and extracts asignal component mapped to the identified resources from the receivedsignals. The DCI format information herein is the one used for theinstruction to transmit A-SRS. To put it more specifically, receptionprocessing section 108 receives A-SRS on the identified resourcesdescribed above in the first common SRS subframe located at or after thefourth subframe from the subframe in which the trigger information istransmitted.

In a case where the received signals are spatially multiplexed signals(i.e., multi-codeword (CW) transmission), reception processing section108 demultiplexes the signals for each CW. In addition, in a case wherethe received signals are OFDM signals, reception processing section 108performs an inverse discrete Fourier transform (IDFT) on the extractedsignal component to transform the OFDM signals into time-domain signals.

The uplink data signals and ACK/NACK information extracted by receptionprocessing section 108 as described above are outputted to datareceiving section 109 while A-SRS signals are outputted to SRS receivingsection 110.

Data receiving section 109 decodes the signals received from receptionprocessing section 108. The uplink data and ACK/NACK information arethus obtained.

SRS receiving section 110 measures reception quality of each frequencyresource on the basis of A-SRS signals received from receptionprocessing section 108 and outputs the reception quality information.When a plurality of A-SRS signals to be transmitted from differentterminals 200 are code-multiplexed using orthogonal sequences and/or thelike, SRS receiving section 110 also demultiplexes the code-multiplexedplurality of A-SRS signals.

(Configuration of Terminal 200)

FIG. 4 is a block diagram illustrating the configuration of terminal 200according to Embodiment 1 of the claimed invention. Terminal 200 hereinis an LTE-A compliant terminal.

In FIG. 4, terminal 200 includes antenna 201, RF receiving section 202,a reception processing section 203, reference signal generating section204, data signal generating section 205, transmission controllingsection 206, transmission signal forming section 207 and RF transmittingsection 208.

RF receiving section 202 performs radio reception processing (e.g.,down-conversion, analog to digital (A/D) conversion and/or the like) onthe radio signals received via antenna 201 and outputs the obtainedreceived signals to reception processing section 203.

Reception processing section 203 extracts the configuration information,assignment control information, trigger information and data signalsincluded in the reception signals.

Reception processing section 203 outputs the configuration information,assignment control information and trigger information to transmissioncontrolling section 206.

In addition, reception processing section 203 outputs the formatidentification information on the DCI from which the trigger informationhas been extracted to transmission controlling section 206. Moreover,reception processing section 203 performs error detection processing onthe extracted data signals and outputs ACK/NACK information inaccordance with the result of error detection to data signal generatingsection 205.

Upon receipt of an instruction to generate reference signals fromtransmission controlling section 206, reference signal generatingsection 204 generates and output reference signals to transmissionsignal forming section 207.

Data signal generating section 205 receives the ACK/NACK information andtransmission data and generates data signals by encoding and modulatingthe ACK/NACK information and transmission data on the basis of MCSinformation received from transmission controlling section 206.

For non-MIMO transmission, data signals are generated using a singlecodeword (CW) while data signals are generated using two codewords forMIMO transmission. It should be noted that, data signal generatingsection 205 also performs CP removal processing and FFT processing whenthe received signals are OFDM signals.

Transmission controlling section 206 configures the resources to whichterminal 200 maps A-SRS signals. To put it more specifically,transmission controlling section 206 identifies the resources on thebasis of the configuration information (i.e., A-SRS transmission ruleconfiguration information) and the format identification information onthe DCI from which the trigger information has been extracted. Theconfiguration information and the format identification information onthe DCI are received from reception processing section 203.Incidentally, how the SRS mapping resources are identified will bedescribed in detail, hereinafter.

Transmission controlling section 206 configures the first common SRSsubframe located after the fourth subframe from the subframe in whichthe trigger information is transmitted, as the transmission subframe forA-SRS. Upon receipt of the trigger information, transmission controllingsection 206 outputs an instruction to generate reference signals toreference signal generating section 204 and also outputs the informationabout the identified SRS resources described above to transmissionsignal forming section 207.

Transmission controlling section 206 identifies “data mapping resources”to which data signals are mapped, on the basis of the assignment controlinformation to be received from reception processing section 203.Transmission controlling section 206 outputs information about the datamapping resources (hereinafter, may be referred to as “data mappingresource information”) to transmission signal forming section 207 andalso outputs MCS information included in the assignment controlinformation to data signal generating section 205.

Transmission signal forming section 207 maps the A-SRS signals receivedfrom reference signal generating section 204 to the SRS mappingresources. In addition, transmission signal forming section 207 maps thedata signals received from data signal generating section 205 to thedata mapping resources indicated by the data mapping resourceinformation. Transmission signals are generated in the manner describedabove. It should be noted that, for Non-MIMO transmission, singlecodeword data signals are assigned to a single layer while two codeworddata signals are assigned to a plurality of layers for MIMOtransmissions. Meanwhile, when the transmission signals are OFDMsignals, transmission signal forming section 207 performs a discreteFourier transform (DFT) on the data signals and maps the resultant datasignals to the data mapping resources. Furthermore, CP is added to thetransmission signals thus formed.

RF transmitting section 208 performs radio transmission processing(e.g., up-conversion, digital to analog (D/A) conversion and/or thelike) and thereafter transmits the processed signals via antenna 201.

(Operations of Base Station 100 and Terminal 200)

A description will be provided regarding operations of base station 100and terminal 200 respectively including the abovementionedconfigurations. The description will be provided herein regarding a casewhere base station 100 uses DCI format 0 as the format of uplinkresource assignment control information and also uses DCI format 1A asthe format of downlink resource assignment control information.

In base station 100, configuration section 101 configures A-SRStransmission rule configuration information for configuration targetterminal 200. In the A-SRS transmission rule configuration information,pieces of identification information on the plurality of controlinformation formats (i.e., DCI formats) are associated with resourceconfiguration numbers defining A-SRS resources corresponding to thepieces of identification information on the respective controlinformation formats. In this embodiment, the assumption is made that theplurality of control information formats are DCI format 0 and DCIformation 1A. Thus, the rules for A-SRS transmission can be provided inthe table illustrated in FIG. 5, for example. In FIG. 5, the first SRSresources associated with DCI format 0 and the second SRS resourcesassociated with DCI format 1A differ in only cyclic shift among a set ofparameters that identify the resources. To put it more specifically,cyclic shift 0 is configured in the resource configuration number (i.e.,SRS resource configuration 1) defining the first SRS resources, whilecyclic shift 6 is configured in the resource configuration number (i.e.,SRS resource configuration 2) defining the second SRS resources. Itshould be noted that, although cyclic shift differs between the firstSRS resources and the second SRS resources n this case, the parameterthat differs therebetween is not limited to cyclic shift. For example,comb number alone or both comb number and cyclic shift may differbetween the first SRS resources and the second SRS resources.Alternatively, bandwidth may differ between the first SRS resources andthe second SRS resources. Base station 100 reports the A-SRStransmission rule configuration information to terminal 200, using RRCsignaling. For example, the A-SRS transmission rule configurationinformation is included in “Sounding RS-UL-Config” message and reportedby the message.

FIG. 6 is a diagram provided for describing transmission of triggerinformation and A-SRS transmission. As the intervals for assignment ofcommon SRS resources, a period of 10 ms (i.e., 10 subframes) isconfigured in FIG. 6. If the assignment period for common SRS resourcesis short, SRS can be transmitted frequently although the resources fordata transmission decrease (i.e., increase in overhead), which in turnleads to a decrease in throughput. For this reason, relatively longintervals are configured as the assignment period for common SRSresources in general.

In a situation where use of A-SRS is more appropriate (e.g., situationwhere a large amount of video data is uploaded in a short period),TCP-ACK and/or the like for uplink data occurs in the downlink. For thisreason, it is likely that both of the uplink resource assignment controlinformation and the downlink resource assignment control information aretransmitted from base station 100 to terminal 200 within 10 ms, which isthe assignment period of common SRS resources. When there is uplink datato be transmitted, the uplink resource assignment control information inDCI format 0 is transmitted, and when there is downlink data to betransmitted, the downlink resource assignment control information in DCIformat 1A is transmitted. Although FIG. 6 illustrates, for the purposeof convenience, a case where uplink resource assignment controlinformation and downlink resource assignment control information aretransmitted in different subframes, uplink resource assignment controlinformation and downlink resource assignment control information can betransmitted in the same subframe.

Accordingly, base station 100 includes trigger information in thedownlink or uplink assignment control information (i.e., downlinkresource assignment control information or uplink resource assignmentcontrol information) and thereby transmits the assignment controlinformation to terminal 200 within a period of 10 ms, which is theassignment period of common SRS resources. Base station 100 can therebycause terminal 200 to transmit A-SRS in the first common SRS subframe ator after the transmission timing of the assignment control information.

Transmission controlling section 206 identifies the SRS mappingresources in terminal 200 on the basis of the A-SRS transmission ruleconfiguration information and the format identification information onthe DCI from which the trigger information has been extracted. A-SRStransmission rule configuration information is reported to terminal 200from base station 100 in advance and is thus shard between base station100 and terminal 200.

In a case where the A-SRS transmission rule configuration informationillustrated in FIG. 5 is reported to terminal 200 from base station 100,A-SRS is mapped to the first SRS resources defined by SRS resourceconfiguration 1 described above, when the format identificationinformation on DCI indicates DCI format 0. Meanwhile A-SRS is mapped tothe second SRS resources defined by SRS resource configuration 2described above, when the format identification information on DCIindicates DCI format 1A in this case.

According to Embodiment 1 as described above, base station 100appropriately selects an assignment control information format (i.e.,DCI format) to include the trigger information therein, when triggeringA-SRS transmission from a plurality of terminals 200, or a periodic SRSis transmitted from another terminal. Accordingly, A-SRS resources usedin each terminal 200 can be flexibly configured. To put it morespecifically, it is possible to flexibly configure the resources usedfor transmission of reference signals while limiting an increase in thenumber of bits used for trigger information for the transmission ofreference signals. As a result, it is possible to avoid a collisionbetween SRS resources of terminals as much as possible while preventinga decrease in throughput due to a delay of SRS.

In addition, since trigger information can be included in any of uplinkresource assignment control information and downlink resource assignmentcontrol information, the trigger information can be reported during bothuplink data assignment and downlink data assignment. Accordingly,appropriately selecting one of the data assignment timings used forreporting an instruction to transmit A-SRS allows flexibly controllingA-SRS resources and also reducing the probability of a collision betweenthe A-SRS resources.

Even when a DCI format other than DCI format 1A is used as the downlinkDCI format, the same effects as those described above can be obtained.In addition, different SRS resources may be configured between DCIformat 0/1A and another DCI format.

Meanwhile, downlink data in a situation where A-SRS is transmitted in aconcentrated manner due to occurrence of uplink data transmissions inbursts in a short period is small in size such as TCP-ACK in many cases.For this reason, the assignment used to report trigger information islimited to smaller contiguous VRBs (or RBs) in this case and assignmentusing DCI format 1A corresponding to a small amount of controlinformation is appropriate in this case. For this reason, A-SRS triggerinformation is reported using only DCI format 0 and DCI format 1A, whichare adjusted to the same size, and no trigger information is added toanother DCI format. Thus, the overhead for data transmission can bereduced.

Embodiment 2

Embodiment 2 relates to cases where trigger information for A-SRS isrepresented by multiple bits.

FIG. 7 illustrates A-SRS transmission rule configuration informationwhen trigger information for A-SRS is represented by two bits. In thisA-SRS transmission rule configuration information, pieces ofidentification information on a plurality of control information formats(i.e., DCI formats) are associated with pieces of information aboutA-SRS resources corresponding to the pieces of identificationinformation for the respective control information formats. In thisA-SRS transmission rule configuration information, there are four bitpatterns depending on the values and combinations of the multiple bits.Accordingly, four sets of SRS resources corresponding to the four bitpatterns are associated with the pieces of identification information onthe control information formats. In addition, a correspondence betweenthe combinations of bit values and the sets of SRS resources associatedwith the combinations of bit values is different at least in partbetween the control information formats. In FIG. 7, the four sets of SRSresources include a set of SRS resources corresponding to the bitpattern (00) (i.e., no A-SRS transmission), a set of SRS resourcescorresponding the bit pattern (01) (i.e., transmission using SRSresource configuration 1), a set of SRS resources corresponding the bitpattern (10) (i.e., transmission using SRS resource configuration 2) anda set of SRS resources corresponding the bit pattern (11) (i.e.,transmission using SRS resource configuration 3). In FIG. 7, the sets ofSRS resources corresponding to the bit patterns other than the bitpatterns 00 are mutually different between DCI format 0 and DCI format1A. Representing trigger information for A-SRS by two bits can increasethe number of options in SRS resource assignment, thus reducing theprobability of a collision between A-SRS resources of terminals.

FIG. 8 illustrates a variation of the A-SRS transmission ruleconfiguration information. In FIG. 8, a correspondence between thecombinations of bit values and the sets of SRS resources associated withthe combinations of bit values is different at least in part between thecontrol information formats. Base station 100 may configure only the setof SRS resources corresponding to the combination of bit values that hasa partially different correspondence between the control informationformats for configuration target terminal 200, individually. In thiscase, the number of candidates for sets of SRS resources reportable asthe trigger information for A-SRS can be kept at minimum. Thus,complexity of terminals 200 and man-hour for testing during developmentof terminals 200 can be reduced.

Embodiment 3

Embodiment 3 relates to cases where so called carrier-aggregation isapplied to communication systems.

In LTE-A systems, the bandwidth is divided into the units of bandwidthcalled “component carriers” each having a bandwidth not greater than 20MHz, which is the bandwidth supported by LTE systems. Such componentcarriers are formed for the purpose of simultaneously achieving backwardcompatibility for LTE systems and communications at an ultrafasttransmission rate, which is several times faster than the transmissionrate in LTE systems. To put it more specifically, “component carrier” isa bandwidth of 20 MHz at maximum and is defined as the basic unit ofcommunication bandwidth. Moreover, “component carriers” in downlink(hereinafter, referred to as “downlink component carriers”) are definedas bandwidths resulting from division based on downlink frequencybandwidth information in BCH broadcasted from a base station or asbandwidths defined by a dispersion width when physical downlink controlchannels (PDCCH) are assigned in the frequency domain in a dispersedmanner. In addition, “component carriers” in uplink (hereinafter,referred to as “uplink component carriers”) are defined as bandwidthsresulting from division based on uplink frequency band information inBCH broadcasted from a base station or are each defined as the basicunit of a communication bandwidth which is not greater than 20 MHz andwhich includes physical uplink shared channel (PUSCH) in the vicinity ofthe center of the bandwidth and PUCCH for LTE on both ends of thebandwidth. The term “component carriers” may be referred to as cells inEnglish in 3GPP LTE-Advanced. LTE-A systems support communications usinga bandwidth obtained by aggregating several component carriers, socalled carrier aggregation.

When so called carrier aggregation is applied to a communication system,identification information on a component carrier (CC) can be includedas a parameter to define an SRS resource. To put it more specifically,pieces of identification information on control information formats(i.e., DCI formats) can be associated with pieces of CC identificationinformation corresponding to the pieces of identification information onthe control information formats in A-SRS transmission rule configurationinformation.

For example, while an SRS resource on a CC on which uplink resourceassignment control information is transmitted (i.e., uplink CC which isthe assignment target of the uplink resource assignment controlinformation) is associated with DCI format 0, an SRS resource on a CCother than a CC on which downlink resource assignment controlinformation is transmitted (i.e., downlink CC which is the assignmenttarget of the downlink resource assignment control information) can beassociated with DCI format 1A.

In this case, base station 100 previously configures the CCidentification information associated with DCI format 1A forconfiguration target terminal 200, and also reports the CCidentification information to configuration target terminal 200 by RRCsignaling in advance. Accordingly, it is possible to flexibly configurethe resources used for transmission of reference signals while limitingan increase in the number of bits used for trigger information for thetransmission of reference signals.

In the abovementioned example, an assumption is made that the CC onwhich the resource assignment control information is transmitted isidentical to the CC which is the assignment target of the resourceassignment control information. However, the embodiment is not limitedto this case. A first CC on which the resource assignment controlinformation is transmitted may be associated with a second CC which isdifferent from the first CC, as the assignment target CC of the resourceassignment control information. In this configuration, while an SRSresource in an uplink CC is associated with a downlink CC on which theuplink resource assignment control information is transmitted, an SRSresource in an uplink CC other than the uplink CC associated with thedownlink CC on which the downlink resource assignment controlinformation is transmitted can be associated with DCI format 1A.

It should be noted that, base station 100 may previously configure thepieces of CC identification information respectively associated with DCIformats 0 and 1A for configuration target terminal 200, and also reportthe pieces of CC identification information to configuration targetterminal 200 by RRC signaling in advance.

In addition, a CC set consisting of a plurality of CCs is configured inany terminal in carrier aggregation. When the amount of data to betransmitted is small, a CC or some CCs in the CC set can be temporarilydeactivated. When deactivating a CC, base station 100 reports the CCdeactivation to terminal 200 by MAC signaling. In this case, it isnecessary to cause terminal 200 to transmit SRS even on the deactivatedCC in order for base station 100 to know the propagation path conditionof the CC. However, terminal 200 receives no PDCCH (i.e., DCI) on adeactivated CC. For this reason, A-SRS transmission cannot be triggeredon a deactivated CC when the assumption is made that the CC on which theresource assignment control information is transmitted is identical tothe assignment target CC of the resource assignment control information.Accordingly, while an SRS resource on a CC which is activated and onwhich the resource assignment control information is transmitted isassociated with DCI format 0, an SRS resource on a deactivated CC otherthan a CC which is activated and on which the downlink resourceassignment control information is transmitted (i.e., downlink CC whichis the assignment target of the downlink resource assignment controlinformation) can be associated with DCI format 1A.

Embodiment 4

In Embodiment 4, downlink resource assignment control information (i.e.,DCI format 1A) is transmitted in a subframe having no downlink data tobe transmitted, and trigger information is included in this downlinkresource assignment control information and transmitted with thedownlink resource assignment control information. The techniquedescribed in Embodiment 4 can be applied to Embodiments 1 to 3 and 5 tobe described, hereinafter.

In the downlink resource assignment control information (i.e., DCIformat 1A) transmitted in a subframe having no downlink data to betransmitted, the value of a predetermined parameter normally included indownlink resource assignment control information is set to apredetermined value, and trigger information indicating an instructionto transmit SRS is also included. Terminal 200 that receives thisdownlink resource assignment control information can recognize thedownlink resource assignment control information as control informationindicating only a trigger for SRS transmission, when the value of thepredetermined parameter is the predetermined value, and the triggerinformation indicating an instruction to transmit SRS is also included.As a combination of the predetermined parameter and its value, theparameter for the number of RBs to be configured, and the maximum numberof RBs or the number of RBs equal to or greater than a threshold can beused, for example. Advantageous effects with the configuration describedabove will be described, hereinafter.

When the amount of downlink data to be transmitted is small, theopportunity to report assignment, using downlink resource assignmentcontrol information (i.e., DCI format 1A) also decreases, so that theremay be no opportunity to report assignment, using downlink resourceassignment control information (i.e., DCI format 1A) until the nearestSRS subframe. In this case, even if there is no downlink data, basestation 200 can appropriately select the format of assignment controlinformation (i.e., DCI format) for including trigger information becausebase station 200 is allowed to transmit the downlink resource assignmentcontrol information in which the value of a predetermined parameternormally included in downlink resource assignment control information isset to a predetermined value, and the trigger information is alsoincluded (i.e., DCI format 1A). Accordingly, A-SRS resources used ineach terminal 200 can be flexibly configured. Accordingly, it ispossible to flexibly configure the resources used for transmission ofreference signals, while limiting an increase in the number of bits usedfor trigger information for the transmission of reference signals. As aresult, it is possible to avoid a collision between SRS resources ofterminals as much as possible while preventing a decrease in throughputdue to a delay of SRS.

Meanwhile, A-SRS is often used in occurrence of a large amount of uplinkdata transmissions in bursts (e.g., uploading video data and/or thelike). For this reason, downlink data that occurs in this case is oftenTCP-ACK and/or the like and is small in size. Accordingly, theopportunities to assign a large number of RBs using downlink resourceassignment control information (i.e., DCI format 1A) are few. Thus, evenwhen the number of RBs to be configured and the maximum number of RBs ora predetermined threshold are set as the predetermined parameter and itsvalue, the flexibility in assignment of RBs in downlink is not affectedsignificantly.

Likewise, only a trigger for A-SRS can be reported using the uplinkresource assignment control information (i.e., DCI format 0) by settingthe value of a predetermined parameter normally included in the uplinkresource assignment control information to a predetermined value andincluding the trigger information in the uplink resource assignmentcontrol information. Normally, there is uplink data in a situation whereA-SRS transmission is to be triggered, however. For this reason, it israre that no uplink data assignment is performed in such a situation.Thus, implementing the function to report only trigger informationwithout reporting resource assignment for data only with downlinkresource assignment control information (i.e., DCI format 1A) achievessimplification of base station 100 and terminal 200 as well as areduction in man-hour for testing during development.

It should be noted that, when invalid assignment (e.g., the number ofRBs not less than the system bandwidth) is reported in the downlinkresource assignment control information (i.e., DCI format 1A), only atrigger for A-SRS may be reported. To put it more specifically, thecombination of a predetermined parameter and its value may be set to acombination of a predetermined parameter and a value that is not validfor the parameter. In addition, as the combination of a predeterminedparameter and its value, a parameter for the MCS level and its maximumor a value not less than a predetermined threshold may be set.Furthermore, when trigger information for SRS, which is represented byat least two bits, is used, only one of the states represented by thebits (e.g., 11) may be associated with a trigger for A-SRS.

When the downlink resource assignment control information (i.e., DCIformat 1A) only indicates a trigger for A-SRS transmission withoutindicating resource assignment for data, the information in the DCIexcept for the information on the trigger for SRS may be all ignored,and a field not related to the downlink data assignment (e.g.,transmission power control information on uplink control channel) may beset as an effective field.

Embodiment 5

Embodiment 5 relates to cases where MIMO can be applied to uplink.

FIG. 9 illustrates A-SRS transmission rule configuration information ina case where MIMO can be applied to uplink. In this A-SRS transmissionrule configuration information, pieces of identification information ona plurality of control information formats (i.e., DCI formats) areassociated with pieces of information about sets of A-SRS resourcescorresponding to the pieces of identification information for therespective control information formats. In Embodiment 5, however, piecesof identification information on a plurality of control informationformats (i.e., DCI formats) are associated with MIMO (or non-MIMO)transmission methods corresponding to the pieces of identificationinformation for the respective control information formats. To put itmore specifically, an A-SRS transmission using a single antenna isassociated with DCI format 0, and A-SRS transmissions for the number ofantennas configured in data transmission (i.e., A-SRS transmission usingMIMO) are associated with DCI format 1A and DCI format 4 in FIG. 9.

There are cases where base station 100 reports a single antennatransmission to terminal 200 as a fallback transmission even forterminal 200 in which uplink MIMO transmission is configured as thetransmission mode. For example, such a case occurs when an increase inerror rate in MIMO transmissions is expected due to a sharpdeterioration in the quality of a propagation path. For this reason,A-SRS transmissions using a single antenna are always associated withDCI format 0 so that A-SRS transmissions using a single antenna can betriggered. In addition, in order to allow base station 100 to measurethe reception quality for a plurality of antennas for MIMO transmission,A-SRS transmissions corresponding to the number of transmission antennasin terminal 200 in which a MIMO transmission mode is configured areassociated with DCI format 4. A-SRS transmissions corresponding to thenumber of transmission antennas in terminal 200 in which a MIMOtransmission mode is configured are associated with DCI format 1A. Thisis because, since A-SRS transmissions for a plurality of antennas aremore likely to involve a collision with an SRS resource for anotherterminal than A-SRS transmissions for a single antenna, the assignmentof SRS resources is made more flexible for the MIMO transmission modethan for the single antenna transmission mode. Accordingly, a greatereffect in reducing the probability of collisions can be obtained.

As described above, it is possible to trigger any of an A-SRStransmission using a single antenna and A-SRS transmissions using aplurality of antennas even for terminal 200 in which an uplink MIMOtransmission is configured as the transmission mode, while limiting anincrease in the number of bits used for trigger information for thetransmission of reference signals. In addition, the effect of reducingthe probability of collisions can be further improved by associating SRSresources for A-SRS transmissions using multiple antennas with a largernumber of DCI formats.

Other Embodiments

(1) In each of the embodiments described above, the parameters definingthe SRS resources include cyclic shift, comb, the number of RBs (orbandwidth), RB position (or SRS frequency domain starting position inthe frequency), frequency hopping pattern, the number of antennas andthe like. Comb herein refers to a signal pattern in signals that has acomb-tooth shaped transmission waveform in the frequency domain (e.g.,waveform having only even numbered subcarriers in the signal component),which is generated when single carrier signals are repeatedlytransmitted. For example, when single-carrier signals are repeatedlytransmitted twice, a waveform of alternate subcarriers is formed, sothat comb number 0 represents an even numbered subcarrier and combnumber 1 represents an odd numbered subcarrier. Meanwhile, comb iscalled the number of repetitions in some cases.

(2) In each of the embodiments, when carrier aggregation is applied to acommunication system, the parameters defining the SRS resources mayinclude information on component carriers. Component carriers are calledcells. In addition, a set of CCs includes one primary cell (PCell) andone or more secondary cells (SCell). In this case, an A-SRS transmissionin PCell is associated with DCI format 0, and a trigger for A-SRStransmission in SCell may be associated with DCI format 1A.

(3) In each of the embodiments, the frequency domain starting position,bandwidth, cyclic shift and comb number are used as the basicconfiguration parameters of each SRS resource configuration, but theparameters are not limited to these parameters and a parameter otherthan these parameters may be included in the basic configurationparameters for SRS resources. All of these basic configurationparameters, i.e., the SRS resource configuration itself may beassociated with each DCI format. Alternatively, only a part of the basicconfiguration parameters may be associated with each DCI format.

(4) In each of the embodiments, it is also possible to additionallyconfigure A-SRS resources to be used when the instructions to transmitSRS are triggered simultaneously in a plurality of DCI formats.Accordingly, even more flexible SRS resource assignment is possible.Meanwhile, an error in receiving PDCCH occurs with a poor error rate inreceiving DCI. When no DCI is detected, A-SRS is transmitted witherroneous SRS resources. For this reason, in a system that may involvean error in receiving PDCCH, terminal 200 may be configured to treat DCIas invalid when receiving trigger information for SRS, which indicatesan instruction to transmit SRS, in a plurality of DCI formats in asingle subframe. This configuration prevents terminal 200 fromerroneously transmitting SRS.

(5) In each of the embodiments, base station 100 may configure whetheror not to include trigger information for SRS in DCI for each terminal200 and report the result of configuration to each terminal 200 by RRCsignaling. In this case, it is possible to reduce the number of bits ofDCI transmitted to terminal 200 in an operation that uses no A-SRS(e.g., voice communications only), or terminal 200 using an applicationthat uses no A-SRS. Accordingly, it is possible to reduce the overheadin this case. Moreover, base station 100 may configure the number ofbits to represent the trigger information for SRS and reports the resultof configuration to terminal 200 by RRC signaling.

(6) In each of the embodiments, terminal 200 is configured to transmitA-SRS in a common SRS subframe. However, the claimed invention is notlimited to this configuration, and terminal 200 may be configured totransmit A-SRS in an individual SRS subframe.

(7) In addition to the parameters for SRS resources, a correspondencebetween information about the transmission power of SRS and each DCIformat may be configured. For example, in a system configured to performinterference control in coordination between cells, A-SRS is triggeredby a DCI format associated with a low transmission power configurationin a subframe whose interference to a different cell is hoped to bereduced, while A-SRS is triggered by a DCI format associated with a lowtransmission power configuration in a subframe whose interference to adifferent cell may be large. Accordingly, it is possible to flexiblyconfigure the transmission power of A-SRS without increasing controlinformation.

(8) SRS transmitted from terminal 200 may be used for downlink weighting(or precoding) control of each antenna and/or the like other than forestimation of a propagation path condition, uplink MSC configuration,frequency scheduling and weighting (directivity) control of each antennaperformed by base station 100. In this case, SRS resources for theuplink MCS configuration, frequency scheduling and weighting control ofantennas, and SRS resources for the downlink weighting control ofantennas may be configured for different DCI formats. Accordingly, it ispossible to trigger A-SRS for each application without increasing thereporting bits.

(9) In each of the embodiments, a description has been provided withantennas, but the claimed invention can be applied to antenna ports inthe same manner.

The term “antenna port” refers to a logical antenna including one ormore physical antennas. In other words, the term “antenna port” does notnecessarily refer to a single physical antenna, and may sometimes referto an antenna array including a plurality of antennas, and/or the like.

For example, 3GPP LTE does not specify the number of physical antennasforming an antenna port, but specifies an antenna port as a minimum unitallowing base stations to transmit different reference signals.

In addition, an antenna port may be specified as a minimum unit to bemultiplied by a precoding vector weighting.

(10) In the each of the embodiments, a description has been providedusing an example in which the claimed invention is implemented byhardware, but the claimed invention may also be implemented by softwarein cooperation with hardware.

The functional blocks used in the description of each of the embodimentsmay typically be implemented as an LSI, which is an integrated circuit.The functional blocks may be formed as individual chips, or some of orall of the functional blocks may be integrated into a single chip. Theterm “LSI” is used herein, but the terms “IC”, “system LSI”, “superLSI”, or “ultra LSI” may also be adopted depending on the degree ofintegration.

In addition, the circuit integration does not have to be achieved usingan LSI and may be achieved using a dedicated circuit or ageneral-purpose processor other than an LSI.

A field programmable gate array (FPGA), which is programmable after LSImanufacturing, or a reconfigurable processor which allowsreconfiguration of connections and settings of circuit cells in an LSIafter LSI manufacturing may be used.

Should a circuit integration technology replacing LSI appear as a resultof advancements in semiconductor technology or other derivativetechnology, the functional blocks could be integrated using such atechnology. Biotechnology applications, and/or the like, are conceivableprospects.

The disclosure of the specification, the drawings, and the abstractincluded in Japanese Patent Application No. 2010-229905, filed on Oct.12, 2010, is incorporated herein by reference in its entirety.

INDUSTRIAL APPLICABILITY

The communication apparatus and communication method of the claimedinvention are useful in that they allow flexibly configuring resourcesused for transmission of reference signals while limiting an increase inthe number of bits used to request the transmission of referencesignals.

REFERENCE SIGNS LIST

-   100 Base station-   101 Configuration section-   102, 103 Coding and modulation section-   104 Transmission processing section-   105, 208 RF transmitting section-   106, 201 antenna-   107, 202 RF receiving section-   108, 203 reception processing section-   109 Data receiving section-   110 SRS receiving section-   200 Terminal-   204 Reference signal generating section-   205 Data signal generating section-   206 Transmission controlling section-   207 Transmission signal forming section

The invention claimed is:
 1. A communication apparatus comprising: areceiver, which, in operation, receives downlink control informationincluding a sounding reference signal (SRS) transmission request, usingone downlink control information format (DCI format) among a pluralityof DCI formats, wherein each DCI format corresponds to at least one setof SRS parameters, at least one set of SRS parameters for one DCI formatis different from another at least one set of SRS parameters for anotherDCI format, and each set of SRS parameters specifies one of a pluralityof SRS resources for use in transmitting the SRS, and wherein theplurality of DCI formats include a first DCI format corresponding to oneset of SRS parameters and a second DCI format corresponding to threesets of SRS parameters; and a transmitter, which, in operation,transmits the SRS using the SRS resource specified by the set of SRSparameters corresponding to said one DCI format.
 2. The communicationapparatus according to claim 1, wherein the set of SRS parametersincludes an initial resource block position of the SRS resource, as wellas a bandwidth, a cyclic shift and a transmission comb of the SRSresource.
 3. The communication apparatus according to claim 1, whereincorresponding relationships between each of the plurality of DCI formatsand the at least one set of SRS parameters are shared with a basestation apparatus.
 4. The communication apparatus according to claim 1,wherein the plurality of SRS resources are resources in an SRS subframeused for transmitting the SRS.
 5. The communication apparatus accordingto claim 1, wherein the transmitter does not transmit the SRS when thereceiving section receives the downlink control information using aplurality of DCI formats in a single subframe.
 6. A communication methodcomprising: receiving downlink control information including a soundingreference signal (SRS) transmission request, using one downlink controlinformation format (DCI format) among a plurality of DCI formats,wherein each DCI format corresponds to at least one set of SRSparameters, the at least one set of SRS parameters for one DCI format isdifferent from another at least one set of SRS parameters for anotherDCI format, and each set of SRS parameters specifies one of a pluralityof SRS resources for use in transmitting the SRS, and wherein theplurality of DCI formats include a first DCI format corresponding to oneset of SRS parameters and a second DCI format corresponding to threesets of SRS parameters; and transmitting the SRS using the SRS resourcespecified by the set of SRS parameters corresponding to said one DCIformat.
 7. The communication method according to claim 6, wherein theset of SRS parameters includes an initial resource block position of theSRS resource, as well as a bandwidth, a cyclic shift and a transmissioncomb of the SRS resource.
 8. The communication method according to claim6, wherein corresponding relationships between each of the plurality ofDCI formats and the at least one set of SRS parameters are sharedbetween a terminal apparatus and a base station apparatus.
 9. Thecommunication method according to claim 6, wherein the plurality of SRSresources are resources in an SRS subframe used for transmitting theSRS.
 10. The communication method according to claim 6, wherein the SRSis not transmitted when the downlink control information is receivedusing a plurality of DCI formats in a single subframe.